Structures using adhesively bonded joints, whether metal-to-metal, metal-to-composite, or composite-to-composite have been used in many design fields, in particular aircraft. The use of adhesives has many advantages, in particular the elimination of fastener holes which improves fatigue life and provides a reduction in fabrication costs.
Many factors affect the quality of adhesively bonded joints, but a primary factor is the surface condition of the parts to be joined. Unfortunately, proper surface condition cannot be easily determined by physical inspection. Thus, one of the major problems with adhesively bonded joints is the determination of whether the bond has an acceptable strength level.
One of the typical ways of determining the quality of the bond is to destructively test test-coupons fabricated along with the part using the same batch of adhesive. Destructive tests are also made on randomly selected production parts. Thus, evaluation of the bonded joints is based on statistical and process control variables. While this procedure is quite satisfactory in many applications, destructive tests can become quite costly, particularly if the part is a large assembly such as a 20.times.40 foot fuselage section of an aircraft.
There are, however, numerous nondestructive inspection techniques that can be used to detect the quality of bonds. For example, the well known ultrasonic inspection techniques wherein a transducer is placed on the surface of the part and the reflected sound wave is monitored. This technique is quite satisfactory for determining the existence of debonds (areas where bonding between the two parts has not occurred). Typical examples of this technique can be found in the following patents: U.S. Pat. No. 2,431,233, "Supersonic Measuring Means," by W. S. Erwin; U.S. Pat. No. 2,499,459, "Resonance Device for Inspecting Materials," by B. Carlin; U.S. Pat. No. 2,522,924, "Supersonic Inspection Apparatus," by N. G. Branson; U.S. Pat. No. 2,549,891, "Supersonic Testing," by B. Carlin; U.S. Pat. No. 2,661,714, "Ultrasonic Gauge," by I. A. Greenwood, Jr., et al; U.S. Pat. No. 2,705,422, "Ultrasonic Inspection Device," by E. A. Henry.
All the prior references cited above are very limited in that they can only accurately locate a flaw in the material, or determine the thickness of the material and debonds. They are not useful for or adapted to determining if the joint can meet minimum acceptable strength levels.
There have, of course, been ultrasonic inspection techniques and equipment developed to test the strength of adhesively bonded joints. For example, U.S. Pat. No. 3,014,364, "Means for Testing Bond Strength," by R. C. Crooks. This process consists of generating an ultrasonic continuous wave signal of substantially constant average energy level for driving a piezoelectric transducer arranged in parallel with an energy measuring circuit. The transducer, when seated on the part to be inspected, exhibits a characteristic impedance which is proportional to the magnitude of restraint against the vibration of the transducer. This impedance level, which is affected among other things by the bond properties of the part under inspection, controls the energy level flowing in the energy measuring circuit. An indication of bond quality may be obtained relative to that exhibited by a similar part of known bond quality.
Here the problem is that this method does not establish bond strength, i.e., a minute area of the joint is not stressed to a minimum acceptable level but rather the effect of the bond on the impedance of the transducer is measured. Additionally, the transducer must make direct contact with the part. Another example of this type of technique can be found in U.S. Pat. No. 2,851,876, "Ultrasonic Apparatus for the Non-destructive Evaluation of Structural Bonds," by J. S. Arnold.
Other patents of interest include U.S. Pat. No. 2,345,679, "Method of Testing Pneumatic Tire Casings," by H. J. Linse; U.S. Pat. No. 2,431,234, "Automatic Sonic Measuring Means," by G. M. Rassweiler, et al; U.S. Pat. No. 2,439,131, "Resonance Inspection Method," by F. A. Firestone; U.S. Pat. No. 2,488,290, "Piezoelectric Crystal Holder," by H. B. Hansell; U.S. Pat. No. 2,494,433, "Transducer Holder," by W. S. Erwin; U.S. Pat. No. 2,618,968, "Supersonic Testing Apparatus," by R. A. McConnell.
Therefore, it is a primary object of this invention to provide a method for determining whether an adhesively bonded joint can meet specified strength requirements.
It is another object of the subject invention to provide a method for determining whether a localized area of an adhesively bonded joint meets minimum specified strength levels by stressing and/or straining the localized area to the minimum specified strength level.
It is a further object of the subject invention to provide a method for determining whether an adhesively bonded joint meets minimum strength requirements by use of either contacting or noncontacting apparatus.